In general, a radio frequency filter (a DR filter, a cavity filter, a waveguide filter, etc.) using a resonator has a kind of circuit tube structure to resonate a radio frequency, particularly, a super high frequency. Since a general resonant circuit including a coil and a condenser has a high radiation loss, the resonant circuit is not suitable to form a super high frequency. The RF filter includes a plurality of resonators, and each resonator forms a metal cylindrical or rectangular cavity surrounded by a conductor. The resonator has a Dielectric Resonance (DR) element or a resonance element including a metal resonant bar within the resonator, and allows only an electromagnetic field of a natural frequency to exist therein, so that the resonator has a structure enabling a resonance of a super high frequency.
As described above, the radio frequency filter using the resonator is largely divided into a Band Pass Filter (BPF) and a band stop filter according a filtering characteristic of the frequency band. At this time, the band stop filter is called a band cutoff filter or a band stop filter.
Such a band stop filter (as well as the band pass filter) has been constantly researched and developed to improve and easily adjust the filtering characteristic, and particularly, active research is also being progressed to restrain a noise generation due to various parasitic resonance modes.